You don't always want the beefiest screw. Sometimes you want it small and cheap. Most screws a random person would have sitting in their garage are of the small and cheap kind, because they're well-suited for most of the everyday projects people do.

For example, I recently mounted speakers to the walls of my living room. The wires come out of the wall, so to make that work well without just having holes in the wall, I mounted outlet boxes in the wall and covered then with panels that let the wire through. Those outlet boxes are screwed into the studs (the boxes have little channels in them to run the screw through). Those screws aren't structural, and probably wouldn't fit if they were.